CD-MUSIC Model Research Articles

Complexation mechanism of Pb2+ on Al-substituted hematite: A modeling study and theoretical calculation

Hematite nanoparticles commonly undergoes isomorphic substitution of Al3+ in nature, while how the Al-substitution-induced morphological change, defective structure and newly generated Al–OH sites affect the adsorption behavior of hematite for contaminants remains poorly understood. Herein, the interfacial reactions between Al-substituted hematite and Pb2+ was investigated via CD-MUSIC modeling and DFT calculations. As the Al content increased from 0% to 9.4%, Al-substitution promoted the proportion of (001) facets and caused Fe vacancies on hematite, which increased the total active site density of hematite from 5.60 to 17.60 sites/nm2. The surface positive charge of hematite significantly increased from 0.096 to 0.418 C/m2 at pH 5.0 due to the increases in site density and proton affinity (logKH) of hematite under Al-substitution. The adsorption amount of hematite for Pb2+ increased from 3.92 to 9.74 mmol/kg at pH 5.0 and 20 μmol/L initial Pb2+ concentration with increasing Al content. More Fe vacancies may lead to a weaker adsorption energy (Ead) of hematite for Pb2+, while the Ead was enhanced at higher Al content. The adsorption affinity (logKPb) of bidentate Pb complexes slightly increased while that of tridentate Pb complexes decreased with increasing Al content due to the presence of ≡ AlOH−0.5 and ≡ Fe2AlO−0.5 sites. Tridentate Pb complexes were dominant species on the surface of pure hematite, while bidentate ones became more dominant with increasing Al content. The obtained model parameters and molecular scale information are of great importance for better describing and predicting the environmental fate of toxic heavy metals in terrestrial and aquatic environments.

Surface properties of schwertmannite with different sulfate contents and its effect on Cr(VI) adsorption

Sulfate is a critical component of schwertmannite. However, the sulfate content of schwertmannite decreases with the increase in AMD-polluted river pH value, and its impact on the surface properties and Cr(VI) adsorption of schwertmannite remains unclear. To address this issue, different methods, including batch adsorption, N2 physisorption, scanning electron microscopy (SEM), potentiometric titration, X-ray photoelectron spectroscopy (XPS), attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR) in combination with two-dimensional correlation spectroscopy (2D-COS) analysis, and surface complexation modeling (SCM) were employed. Our results showed that sulfate existed as both non-protonated bidentate-binuclear complexes and non-protonated outer-sphere complexes in schwertmannite. The surface morphology and particle size of schwertmannite were unaffected by the sulfate content. However, a decrease in sulfate content resulted in a drop in the negative charge on the surface of schwertmannite, which is conducive to particle aggregation. An increase in pH, on the other hand, raised the negative charge and encouraged the dispersion of the particles. And then, reducing the sulfate content of schwertmannite led to a decrease in acid-base buffer and Cr(VI) adsorption capacity of schwertmannite. The adsorption process of Cr(VI) by schwertmannite can be divided into two stages. In the first stage, Cr(VI) was adsorbed primarily through anion exchange reactions with the sulfate; however, the contribution of surface complexation gradually increased as the initial sulfate content decreased because of a reduced likelihood of anion exchange and an increase in the specific surface area of schwertmannite. Additionally, the behavior of sulfate and Cr(VI) in this stage could be predicted based on the initial sulfate content of schwertmannite. In the second stage, surface complexation with hydroxyl sites became the predominant mechanism of adsorption, which still exhibited great potential at higher Cr(VI) concentrations. Finally, the aforementioned hypothesis was used to establish a CD-MUSIC model, and the fitting results demonstrated that the density and Cr(VI) adsorption capacity of the sulfate site dropped as the sulfate content decreased, while the hydroxyl site was less impacted. The findings have provided insights into further understanding the properties of schwertmannite and its adsorption of Cr(VI).

Novel insight into the competitive adsorption behaviors of As(V), Sb(V), and P(V) on {1 1 0} facets of Goethite: Existing form and coordination structure affinity

Despite the geochemical prominence of As(V), Sb(V), and P(V) coexisting in an extensive array of iron-oxide environments, the comprehension of their mobility and destiny is constrained due to the differences in the surface properties and exposed crystal facets of iron oxides. Herein, goethite primarily exposing the {110} facet, was employed as the model iron mineral to investigate the competitive adsorption behavior of As(V), Sb(V), and P(V) via batch experiments leveraging EXAFS, CD-MUSIC model, and DFT calculations. Results showed that observed co-adsorption behaviors were attributed to the comprehensive action of goethite surface chemistry, ion existing forms and different coordination structures stability at the molecular scale. CD-MUSIC model indicated As(V) and P(V) competed for surface sites through a combination of bidentate and monodentate coordination, whereas Sb(V) was primarily adsorbed via bidentate coordination, which led to the surpassed competitiveness of As(V) and P(V) on {110} facet compared to that of Sb(V). The affinity of various coordination structures on {110} facets based on DFT was found to be consistent with the trends predicted by the CD-MUSIC model. Specifically, As(V) and Sb(V) exhibited a greater affinity for bidentate coordination, while P(V) tended to form monodentate coordination complexes. However, the existing forms of As(V), Sb(V), and P(V) under different pH, especially for protonated species, accelerated the more weakened competitive combination ability of Sb(V) than As(V) and P(V) with increasing pH. Furthermore, charge density difference and Bader analysis have confirmed the contribution of electron transfer from the surface Fe to O towards forming FeO bonds. This work provided a novel investigation way and facilitated a more profound understanding of the geochemical cycle of toxic metals.

Transport and Model Calculation of Microplastics Under the Influence of Ionic Type, Strength, and Iron Oxide

Microplastics (MPs) in soil have attracted extensive attention as an emerging pollutant, and the transport of MPs is affected by their own physical and chemical properties, the chemical composition of soil solutions, and soil minerals. However, in the presence of oxides, the underlying mechanism for the transport of MPs in different ionic types and ionic strengths is still not fully understood. In this study, the effects of ionic type, ionic strength, and iron oxide on the transport of polystyrene microplastics (PSMPs) with different functional groups were investigated through stability experiments and transport experiments. The colloid transport model, CD-MUSIC model, and DLVO theory were used to explore the transport mechanism. The results showed that normalized concentrations (c/c0) of PSMPs were 0.99 in the NaH2PO4 background and 0.94 in the CaCl2 background, respectively, which indicated that the strongest stability of PSMPs was observed in the former and the weakest in the latter. Different ionic types had different effects on the transport of PSMPs. For the cations Na+ and Ca2+, Ca2+ strongly inhibited PSMPs transport in pure quartz sand because of the bridging effect and strong charge neutralization effect; the recovery rate of the PSMPs in the effluent was (43.83±1.71)%, and a first-order retention coefficient on the second kinetic Site-2 (k2a) was 1.54 min-1. The presence of iron oxide enhanced the inhibition, the recovery rate of the PSMPs in the effluent decreased to (6.04±0.40)%, and k2a increased to 5.33 min-1. For the anions Cl- and PO43-, the transport of PSMPs in pure quartz sand was dominated by surface electronegativity of PSMPs, and PSMPs exhibited lower electronegativity under Cl- background and thus showed higher recovery[(92.95±0.63)%] and lower k2a (0.19 min-1). However, in the presence of iron oxides, the Zeta potential of the quartz sand surface was the controlling factor for PSMPs transport. According to results of the CD-MUSIC model, PO43- could be easily adsorbed on the iron oxide surface to form innersphere complexes, which reduced the surface electronegativity of the iron-loaded quartz sand and enhanced the transport of PSMPs, higher recovery[(76.22±1.39)%], and lower k2a (0.66 min-1). Moreover, the species of the formed innersphere complex was controlled by the PO43- concentration, and different species of innersphere complexes had distinct negative surface charges. Higher surface electronegativity of the iron-loaded quartz sand was observed under higher PO43- concentration, which was not conducive to the transport of PSMPs. Further, the transport ability of PSMPs decreased with the increase in ionic strength. Finally, the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was used to calculate the variation in the primary barrier between PSMPs and the collector under the conducted experimental conditions, which helped better elucidate the transport behavior of PSMPs. The variation in the primary barrier was consistent with the transport ability of PSMPs, and a higher primary barrier indicated a larger repulsion between PSMPs and the collector, which was in favor of PSMPs transport.

Adsorption behavior of soil fulvic acid on crystal faces of kaolinite and goethite: Described by CD-MUSIC model

The interaction between organic matter (OM) and minerals has significant impacts on the mineralization process and rate of OM, and can protect part of organic carbon in soils. In this study, the complex species of fulvic acid (FA) on the surface of minerals (kaolinite and goethite) and the corresponding thermodynamic characteristics were investigated with the CD-MUSIC model and isothermal titration calorimetry. With increasing pH, the adsorption of kaolinite for FA increased at pH < 5.5 and decreased at pH > 5.5, which might be due to the binding of carboxyl groups of FA onto the positively charged sites of O-face and edge-face. However, that of goethite consistently decreased with increasing pH from 3.5 to 9.0 due to an increase in electrostatic repulsion. The fraction of FA was mainly adsorbed on the edge-face and O-face to form inner sphere complexes on kaolinite and outer sphere complexes on goethite. Molar adsorption enthalpies indicated that FA was adsorbed through two site types on kaolinite, while that was one site type on goethite. In addition, the molar enthalpy was more closely associated with inner sphere complexation for kaolinite, while with outer sphere complexation for goethite. The findings shed new light on the adsorption behavior and complexation mechanism of OM on the crystal faces of mineral-water interface.

Structure and morphology control of amorphous Mn-doped Fe oxide composite using soft-templates for efficient Sb(V) removal in textile wastewater: Mechanism of solid-liquid interface regulation and Sb(V) adsorption

To achieve high removal rate and efficacy of Sb(V) from textile wastewater and simultaneously realize cyclic Sb(V) recovery, the structure and surface property of amorphous Mn-doped Fe oxide composite (MFOC) were regulated via the combination of facile soft-template synthesis and sintering treatment. Characterization and adsorption results indicated MFOC structure was greatly changed from aggregated large sphere to dispersive small sphere after adjusted by the optimal Brij-58 (SMFO-B). Because of the generated fasciculate micelle, long carbon chain feature and the lowest residual amount of Brij-58, abundant mesopores were generated and the aggregation of unit particles was apparently decreased for SMFO-B. Surface area and total pore volume of SMFO-B became 10 and 6 times of MFOC and more functional hydroxyl groups were generated and exposed on the interface between SMFO-B and water solution. Additionally, the surface positive potential of SMFO-B under neutral condition was also greatly increased. Thus, the maximum adsorption capacity and rate of SMFO-B towards Sb(V) reached to 147 mg/g and 0.1067 mg/(g·min), 2 and 5 times of MFOC. SMFO-B also possessed better selective adsorption ability, exhibited stronger static and dynamic adsorption performance in real textile wastewater and realized the cyclic desorption and enrichment of Sb(V) due to its better phase stability. X-ray absorption spectroscopy showed two main specific forms, such as bidentate edge-(E2) and corner-sharing (C2) complexes, for the inner-sphere complexation during Sb(V) adsorption and E2 complex was proved to play the most important role through the analysis of CD-MUSIC model. This study supplied an excellent adsorbent for Sb(V) removal from textile wastewater and a deeper insight about Sb(V) adsorption onto amorphous Fe oxide.

Influencing factors and environmental effects of interactions between goethite and organic matter: A critical review

This paper reviews progresses in the interactions between goethite and organic matter (OM) and their environmental effects in recent decades. The interactions mainly include the effect of organic matter on the surface properties and the crystallization of goethite, molecular changes of OM caused by goethite, and their interaction mechanisms, which can be depicted by the commonly used Langmuir model, the charge distribution multi-site complexation model (CD-MUSIC model), ligand charge distribution model (LCD model), and natural organic matter charge distribution model (NOM-CD model). The influencing factors of the interactions are summarized with emphasis on the external, including pH, ionic strength, carbon dioxide (CO2), and the internal, including the structure of OM and iron species. The goethite—OM complexes caused by the interactions will affect migration and transformation of conventional heavy metals and emerging antibiotics. The complexes, as the carrier of carbon and iron, are also the critical parts of the carbon and iron cycles, which are associated with climate change. This review provides a basis for future mechanism studies of formation, transformation, and effects of goethite—OM complexes (particulate OM or carbon-contained minerals) in different environmental systems at a molecular level.

Dynamic Adsorption of As(V) onto the Porous α-Fe2O3/Fe3O4/C Composite Prepared with Bamboo Bio-Template

Arsenic (As(V)), a highly toxic metalloid, is known to contaminate wastewater and groundwater and is difficult to degrade in nature. However, the development of highly efficient adsorbents, at a low cost for use in practical applications, remains highly challenging. Thus, to investigate the As(V) adsorption mechanism, a novel porous α-Fe2O3/Fe3O4/C composite (PC-Fe/C-B) was prepared, using bamboo side shoots as a bio-template, and the breakthrough performance of the PC-Fe/C-B composite-packed fixed-bed column in As(V) removal was evaluated, using simulated wastewater. The PC-Fe/C-B material accurately retained the hierarchical porous microstructure of the bamboo bio-templates, and the results demonstrated the great potential of PC-Fe/C-B composite, as an effective adsorbent for removing As(V) from wastewater, under the optimal experimental conditions of: influent flow 5.136 mL/min, pH 3, As(V) concentration 20 mg/L, adsorbent particle size &lt; 0.149 mm, adsorption temperature 35 °C, PC-Fe/C-B dose 0.5 g, and breakthrough time 50 min (184 BV), with qe,exp of 21.0 mg/g in the fixed-bed-column system. The CD-MUSIC model was effectively coupled with the transport model, using PHREEQC software, to simulate the reactive transportation of As(V) in the fixed-bed column and to predict the breakthrough curve for column adsorption.

Competitive Arsenate and Phosphate Adsorption on Ferrihydrite as Described by the CD-MUSIC Model

Competitive Arsenate and Phosphate Adsorption on Ferrihydrite as Described by the CD-MUSIC Model

Adsorption of Phosphorus Oxyanions at the FeOOH(goethite)/Water Interface: The Importance of Basicity.

PIII-containing H-phosphonate (HPO32-) and its monoethyl ester (fosetyl), essential pesticides for control of oomycete and fungal pathogens, are among the few pesticides transported by both xylem and phloem, making application as folia spray, soil spray, and trunk injection equally effective. To understand bioavailability and efficacy within soils, knowledge of adsorption to soil minerals is important. FeOOH(goethite) is often selected as an archetypal mineral surface. In the present work, H-phosphonate (with pKa values of 1.5 and 6.78) adsorption onto FeOOH is nearly complete below pH 6 and decreases to negligible amounts by pH 11, following an S-shaped curve. Fosetyl (pKa: 0.9), in contrast, does not adsorb to any significant extent, regardless of pH. To place these observations in context, adsorption of six other phosphorus oxyanions was investigated, and fitted using a CD-MUSIC model. Phosphate defines a similar S-shaped curve but adsorbs more strongly than H-phosphonate. Despite moderate differences in basicity, pH dependence and extents of adsorption for the four additional diprotic oxyanions methylphosphonate (pKas: 2.40, 8.00), benzylphosphonate (2.24, 7.93), phenylphosphonate (1.9, 7.47), and phenyl phosphate (1.1, 6.28) are quite similar to those of H-phosphonate. As with fosetyl, the other low pKa monoprotic oxyanion in our study, phenylphosphinate (pKa: 1.75), does not adsorb. Basicity, that is, pKa, is revealed to be the principal determinant of extents of adsorption.

Insights into the improving mechanism of defect-mediated As(V) adsorption on hematite nanoplates

The fate of As(V) in subsurface environments is strongly affected by ubiquitous iron oxides. Defects are commonly present in natural hematite, while the impacts of defects on the active sites and complexation mechanism of hematite for As(V) remain poorly understood. In this study, the defect-rich hematite was employed to investigate the surface charge characteristics and As(V) adsorption behavior using potentiometric acid-base titration and CD-MUSIC model in comparison with corresponding defect-poor hematite. The total arsenate-active site density (5.7 sites/nm2) on defective hematite includes 1.2 sites/nm2 of original sites and 4.5 sites/nm2 of Fe vacancy-induced sites. The result revealed that the vacant Fe3+ sites in defective hematite was compensated by the protons in solution, thus resulting in a considerable increase in site density as well as positive charge. The CD-MUSIC modeling results demonstrated that the presence of Fe vacancies in hematite is beneficial to the improvement in affinity constants for both monodentate and bidentate arsenate complexes. The high adsorption capacity of defective hematite (2.60 μmol/m2) compared to defect-free hematite (1.33 μmol/m2) is attributed to its large affinity constants as well as its more active surface sites, thereby playing a vital role in reducing the threats of heavy metals in the environment.

Predicting breakthrough of vanadium in fixed-bed absorbent columns with complex groundwater chemistries: A multi-component granular ferric hydroxide−vanadate−arsenate−phosphate−silicic acid system

Granular ferric hydroxide (GFH) is often used for fixed bed adsorbent (FBA) columns in groundwater purification units around the world to remove arsenate contaminations. Groundwater can contain also other toxic (e.g., antimonite and vanadate) and non-toxic oxo-anions (phosphate and silicic acid) that are known to affect FBA lifetimes. Therefore, understanding the breakthrough of toxic compounds intended for removal by FBA is essential to their design, and is important to predict accurately breakthrough curves (BTCs) for FBAs in waterworks to plan future operating costs. Rapid small-scale column tests (RSCCT) and pilot-scale FBA were used to simulate vanadate BTCs for complex groundwater chemistries. The BTCs were simulated successfully using a homogeneous surface diffusion model (HSDM) combining equilibrium chemical adsorption and kinetic mass transfer. Adsorption parameters for various groundwater compositions were predicted using the CD-MUSIC surface complexation model, which was set up for the first time for akaganéite-based granular ferric hydroxide with a competitive multi-solute system. The results indicated that V(V) is less prone to competitive adsorption effects, and use of the homogeneous surface diffusion model to predict the BTCs requires then the kinetic mass transfer Biot number to be used as the only fitting parameter. On the other hand, a concentration overshoot could be observed for the two weaker absorbed oxo-anions arsenate and phosphate because of displacement by the vanadate. Results of pilot scale test column BTCs of vanadate for three waterworks with different groundwater compositions could be favorably extrapolated with a unique Freundlich constant kF of 3.2 derived on basis of the multi-solute CD-MUSIC model, and a unique Biot number of 37 fixed for all three different test sites.

Influence of seawater ions on phosphate adsorption at the surface of hydrous ferric oxide (HFO)

The adsorption of phosphate on hydrated ferric oxide (HFO) was studied in solutions containing major seawater ions (Na+, Mg2+, Cl−, SO42−, Ca2+, K+) at pHs 6.5, 7.5 and 8.5. The presence of these ions promotes phosphate adsorption and the process is electrostatic in nature. Despite this electrostatic force, the precipitation of hydroxyapatite in the presence of Ca2+ at pH 8.5 also plays an important role in the removal of phosphates from the dissolved phase. Energy dispersive X-ray spectroscopy (EDS) and Fourier transform infrared (FTIR) spectra support that phosphate adsorption on HFO surface can be attributed to inner sphere complexes with the formation of bidentate complexes (FeO)2PO2 in the presence of main seawater ions at pH = 8.5. The results of EDS clearly indicated that Fe-P-Ca complexes, Fe-P-Mg, or other phosphate-bridged ternary complexes were not formed during adsorption in the presence of NaCl, KCl, CaCl2, Na2SO4 and MgCl2. This observation differs somewhat from that the typical explanation used to describe the phosphate adsorption mechanism on HFO. The CD-MUSIC model makes it possible to describe this adsorption mechanism of phosphate on HFO in the presence of 0.7 M NaCl, and these outcomes are coherent with the experimental FTIR and EDS results.

Soil organic matter increases antimonate mobility in soil: An Sb(OH)6 sorption and modelling study

The role of organic matter (OM) in antimonate (further denoted as Sb(OH)6) mobility in soil is unclear. The objective of this study was to evaluate Sb(OH)6–OM interaction. Antimonate solid:liquid distribution coefficients (KD) were measured at low Sb concentrations in soil samples with a natural gradient in soil organic carbon (OC) that were collected from different depths of up to 3 m in two excavated soil profiles and in a subset of four soil samples with experimentally increased OM concentration from addition of Suwannee River OM. The KD values were related to soil properties by multiple linear regression and described with the CD–MUSIC model of ferrihydrite. The KD values ranged from 12 to 2800 L kg−1 and decreased strongly with increasing OC concentrations, when normalized to the amount of iron (Fe) and aluminium (Al) in acid oxalate extracts (r = −0.69; p < 0.0001). Experimentally increasing OC by ∼1.5 g kg−1 increased soluble Sb and decreased Sb(OH)6KD values by up to a factor of 8. The multiple regression model reveals that sorption of Sb(OH)6 to Fe and Al hydroxides decreases with increasing pH and increasing dissolved organic carbon concentration. This effect could be explained with geochemical modelling by the competitive and electrostatic effects of adsorbed humic substances on Sb(OH)6 surface complexation to the reactive surface sites of the Fe and Al hydroxides. Finally, both models could predict the in situ pore water Sb concentrations of unspiked samples, with a RMSE of 0.35 for the regression model and 0.43 for the geochemical model on the log10Sb concentrations. For these predictions, the 0.1 M Na2HPO4–extractable Sb concentration was measured and used to estimate the reversibly sorbed Sb pool. This study shows that increasing soil OM increases Sb(OH)6 mobility at low soil Sb concentration, likely due to competitive sorption on Fe and Al hydroxides and a process based, geochemical model was calibrated to describe Sb(OH)6 mobility in soil.

Surface complexation modeling of arsenic mobilization from goethite: Interpretation of an in-situ experiment

Abstract Sorption has been recognized as a predominant process for the mobility and transport of arsenic (As) in groundwater. However, the model-based description of the chemical and physical mechanisms controlling As interaction with mineral surfaces under natural hydrochemical conditions remains a formidable challenge. In this study we assess and compare the ability of existing surface complexation models (SCMs), the diffuse double layer model (DDL) and the charge distribution multisite complexation model (CD-MUSIC), to simulate As desorption from goethite in groundwater. We consider the outcomes of an in-situ experiment recently performed in an arsenic-contaminated aquifer of Northern China where As-loaded goethite-coated sand was deployed in 7 monitoring wells. Determination and measurements of As-surface species were carried out over a time period of 80 days in all monitoring wells. Simultaneous calibration of the models with the measurements in the 7 wells was performed to obtain single sets of surface complexation parameters for the DDL and the CD-MUSIC models, respectively. Although the general dynamic pattern of the As release at the site is captured by both models, the approach based on the CD-MUSIC agrees best with the field experimental data without modifications of the surface complexation database compiled from previous studies on goethite. Conversely, calibration of the DDL affinity constants led to substantial improvement in the agreement between model simulations and the considered field dataset. The model outcomes and the exploration of the sensitivity of the goethite surface composition to changes in hydrochemical conditions provide insights into the mechanisms controlling arsenic sorption and their different description in the DDL and the CD-MUSIC models. Both SCMs indicate that PO43− acts as the main competitor for As(V) sorption sites, and that Fe(II) does not have a significant effect on the As(V) release despite its strong affinity for the goethite surface. Neither SCM suggests direct binding of As(III) to goethite. However, the CD-MUSIC model predicts significant formation of a goethite-Fe(II)-As(III) complex under the mildly alkaline conditions of the groundwater at the field site and that such complex is insensitive to phosphate competition. The CD-MUSIC implementation also allows capturing the non-linear charge effects of the major ions, including C a 2+ and M g 2+ , which appear to have important implication in the mechanisms of As sorption at the field site.

Phosphate speciation on Al-substituted goethite: ATR-FTIR/2D-COS and CD-MUSIC modeling

Natural goethite commonly encounters Al substitution for Fe, which alters the surface reactivity of goethite.

Investigation on the control of phosphate leaching by sorption and colloidal transport: Column studies and multi-surface complexation modelling

Surface complexation modelling (SCM) is a powerful tool to estimate speciation and fate of solutes in soil, provided sufficient model validation. This study aims to describe phosphate (PO4) leaching with SCM. The leachate phosphorus concentrations ([P]) of 120 unsaturated columns of contrasting agricultural soils were measured and modelled. Leachate [P] ranged 0.7–240 μM. Leachate [P] increased as the ratio of P to iron and aluminium (PAl+Fe) in acid oxalate soil extracts increased and as leachate Fe and Al concentrations ([Al + Fe]) increased. SCM was used to describe PO4 sorption to ferrihydrite (CD-MUSIC model). This yielded adequate description of leachate [P] (RMSElog10 = 0.39), but only when reactive PO4 was described from isotopically exchangeable PO4, when organic matter was included as the main competing adsorbate and when mobile colloidal ferrihydrite was included. The model reveals that colloidal PO4 transport enhanced leachate PO4 concentrations up to a factor 50 at small soil P content and small calcium (Ca2+) concentration in solution, as a large Ca2+ concentration enhances colloidal stability. This modelling approach explained that long-term application of organic fertilisers with higher Ca content reduced P leaching, likely due to the effect of Ca2+ on colloidal stability. A two-parameter empirical Langmuir model, based on soil Fe and Al oxyhydroxides, fitted data better than any SCM, suggesting that the empirical model might be advocated for application at large scale. This study revealed the power of SCM to better understand colloidal transport of P in soil.

Influence of calcium and phosphate on pH dependency of arsenite and arsenate adsorption to goethite

In the environment, simultaneous presence of arsenic (As) of different oxidation states is common, which hampers our understanding of As behavior. In the current study, the pH dependency of arsenite (As(III)) and arsenate (As(V)) adsorption to goethite under the influence of calcium (Ca2+) (as a major cation) and phosphate (PO43−) (as a major anion) was studied, and the reliability of the CD-MUSIC model prediction was tested. The results show that the presence of the major ions led in general to a weaker and more complicated pH dependency of As adsorption. Calcium promoted As(V) adsorption especially at high pH, which can reverse the direction of the pH dependency. The presence of Ca2+ can even decrease As(III) adsorption when As(V) and/or PO43− are present. Phosphate competed strongly with both As(III) and As(V) in their adsorption, especially at intermediate and low pH. In the multi-component system, As(III) adsorbs weaker than As(V) over the environmental relevant pH range, therefore it is often the dominant As species in solution and soluble As(III) concentration generally decreases with increasing pH. In the same pH range, As(V) adsorption shows a complicated pH dependency. Soluble As(V) reaches a minimum around pH 6 at high concentration of major bivalent cations (e.g. Ca2+), whereas soluble As(V) will decrease with pH at low bivalent cation concentrations. The experimental results can be reliably predicted and explained with the CD-MUSIC model. The outcome of this study can provide understanding needed in the risk assessment and remediation of As contaminated soils and water.

Selenium(IV) Sorption Onto γ-Al2O3: A Consistent Description of the Surface Speciation by Spectroscopy and Thermodynamic Modeling.

The sorption processes of Se(IV) onto γ-Al2O3 were studied by in situ Infrared spectroscopy, batch sorption studies, zeta potential measurements and surface complexation modeling (SCM) in the pH range from 5 to 10. In situ attenuated total reflection fourier-transform infrared (ATR FT-IR) spectroscopy revealed the predominant formation of a single inner-sphere surface species at the alumina surface, supporting previously reported EXAFS results, irrespective of the presence or absence of atmospherically derived carbonate. The adsorption of Se(IV) decreased with increasing pH, and no impact of the ionic strength was observed in the range from 0.01 to 0.1 mol L-1 NaCl. Inner-sphere surface complexation was also suggested from the shift of the isoelectric point of γ-Al2O3 observed during zeta potential measurements when Se(IV) concentration was 10-4 mol L-1. Based on these qualitative findings, the acid-base surface properties of γ-Al2O3 and the Se(IV) adsorption edges were successfully described using a 1-pK CD-MUSIC model, considering one bidentate surface complex based on previous EXAFS results. The results of competitive sorption experiments suggested that the surface affinity of Se(IV) toward γ-Al2O3 is higher than that of dissolved inorganic carbon (DIC). Nevertheless, from the in situ experiments, we suggest that the presence of DIC might transiently impact the migration of Se(IV) by reducing the number of available sorption sites on mineral surfaces. Consequently, this should be taken into account in predicting the environmental fate of Se(IV).

Phosphate adsorption and precipitation on calcite under calco-carbonic equilibrium condition

Phosphate (PO43−) removal on calcite often entails two processes: adsorption and precipitation. Separating these two processes is of great importance for assessment of PO43− stability after removal. Thus, this study was aimed at finding a critical range of conditions for separating these two processes in calco-carbonic equilibrium, by adjusting PO43− concentration, reaction time and pH. PO43− removal kinetic results showed that: (I) At pH7.7, PO43− removal was mainly by adsorption at initial PO43− concentration ≤2.2 mg L−1 and reaction time ≤24 h, with dominant precipitation occurring at initial PO43− concentration ≥3 mg L−1 after 24 h reaction; (II) At pH8.3, adsorption was the key removal process at initial PO43− concentration ≤7.5 mg L−1 and reaction time ≤24 h, whereas precipitation was observed at initial PO43− concentration of 10 mg L−1 after 24 h reaction, (III) At pH 9.1 and 10.1, PO43− removal mechanism was mainly by adsorption at initial PO43− concentration ≤10 mg L−1 within 24 h reaction. Based on the kinetic results, it is suggested that PO43− precipitation will occur after 24 h reaction when saturation index of amorphous calcium phosphate is between 1.97 and 2.19. Besides, increasing PO43− concentration does not cause a continuous decline of PO43− removal percentage. Moreover, experimental removal data deviated largely from the theoretical adsorption value by CD-MUSIC model. These indicate occurrence of precipitation which is in agreement with the kinetic result. Therefore our study will provide fundamental reference information for better understanding of phosphorous stabilization after removal by calcite.